Directional valve

ABSTRACT

A directional control valve which has a movable control piston ( 5 ) which separates two pressure chambers ( 6, 8 ) from each other in the valve housing interior, which pressure chambers can be subjected to control pressure in order to move the control piston ( 5 ) into axial positions corresponding to relevant switch positions of the valve in accordance with the difference in pressure prevailing between the control chambers ( 6, 8 ). An arrangement ( 29 ) acts on the control piston ( 5 ), produces an actuating force and prestresses the control piston for movement into an axial position corresponding to a desired predetermined position.

FIELD AND BACKGROUND OF THE INVENTION

The invention relates to a directional control valve which has a movablecontrol piston which separates two pressure chambers from each other inthe valve housing interior, which pressure chambers can be subjected tocontrol pressure in order to move the control piston into axialpositions corresponding to relevant switch positions of the valve inaccordance with the difference in pressure prevailing between thecontrol chambers.

Directional control valves of this type are known. For example, DE 19710 318 A1 shows a valve of this type in the form of a three-wayproportional fitted valve having an inductive travel sensor fordetecting the position of the control piston and an added pilot valvevia which the pressure chambers for controlling the position of thecontrol piston can be subjected to control pressure in order to move thecontrol piston into the relevant axial positions which correspond to thedesired switch positions of the valve.

In the case of valves of this type, if, for some reason, no controlpressure acts on the control piston, whether because the system whichthe valve forms part of is switched off and there is no system pressure,or whether a power failure at the pilot valve or emergency trippingcauses the hydraulic short-circuiting of the pressure chambers at thecontrol piston, then the pressure piston takes up an undefined, axialposition. When the system pressure is switched on, a consumer which isconnected may therefore execute an undesired, uncontrolled movement,which may constitute a risk of an accident or may result in a breakdown.

SUMMARY OF THE INVENTION

The invention is based on the object of providing a directional controlvalve in which the above-mentioned problems resulting because of acessation of the control pressure are avoided.

In the case of a directional control valve of the type mentioned at thebeginning, this object is achieved according to the invention by thefact that an arrangement is provided which acts on the control piston,produces an actuating force and prestresses said control piston formovement into an axial position corresponding to a desired predeterminedposition.

This ensures that even when there is no difference in pressure betweenthe pressure chambers which can be subjected to the control pressure,the control piston takes up a position at which the system pressure canbe switched on without any risk and without particular safety measureshaving to be taken.

When there is no difference in pressure in the pressure chambers, thedesired position to be taken up by the control piston preferablycorresponds to a position at which the valve forms a connection betweena consumer connection and a tank connection. In this arrangement, ifthere is a cessation of the control pressure, a consumer which isconnected comes into its final operating position, i.e. it moves to itsend stop.

So that this process takes place without any risk, i.e. the consumeronly moves slowly to its end stop, in a particularly advantageousexemplary embodiment the arrangement is made in such a manner that thearrangement producing the actuating force exerts a force on the controlpiston that counteracts flow forces which act on said control pistonwhen pressure medium flows from the consumer connection to the tankconnection, and that the size of the actuating force is selected to bejust sufficient so that the flow forces, from a certain strength of thepressure-medium flow, compensate for the action of the actuating forceon the control piston. The effect achieved by this is that when there isa lack of difference in the control pressure, the control piston isopened by the actuating force, which acts on it, with the effect ofopening the connection from the consumer connection to the tankconnection only to an extent sufficient for a certain strength of thepressure-medium flow to the tank connection to arise, at which strengththe flow forces acting here on the control piston compensate for theactuating force. As a result, a state of equilibrium arises at a desiredpressure-medium flow, which can be selected by the strength of theactuating force, from the consumer to the tank. With appropriateselection of the strength of the actuating force, which prestresses thecontrol piston, this state of equilibrium corresponds to a strength ofthe pressure-medium flow at which the relevant consumer moves only veryslowly to its end stop.

A spring arrangement can be provided to produce the actuating forceacting on the control piston. In an advantageous exemplary embodiment inwhich the control piston forms, at its front end, a pipe valve which isguided in a valve bushing and whose open, front end is connected to theconsumer connection and which has a control aperture which produces theconnection to the tank connection in an axial position slid forward outof the valve housing, the spring arrangement is formed as a compressionspring which is supported at one end on the tear end of the controlpiston and at the other end on the inner end of the valve housinginterior. The spring force is selected in such a manner that even at alow strength of the flow of the pressure medium flowing into the open,front end of the pipe valve, the flow forces which are effective on thecontrol piston equalize the spring force, so that the above-mentionedstate of equilibrium arises even at a small pressure-medium flow.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is explained in detail below with reference to thedrawing, in which the sole FIGURE shows a longitudinal section, drawn ina schematically simplified manner in part, of an exemplary embodiment ofthe directional control valve according to the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

A fitted directional control valve denoted as a whole by 1 in the FIGUREhas a valve housing 3 in which a control piston 5 can be displaced inthe axial direction, with reference to the longitudinal axis 7 of thehousing, in order to bring about, in a 3/2-way function, a continuouscontrol of a pressure-oil volume flow from the pressure-oil connection Pto the consumer connection A and from the connection A to the tankconnection T. The control piston 5 has a control collar 4 which isenlarged in diameter and partitions off two pressure chambers 6 and 8 inthe housing 3. For actuation of the directional control valve 1, thesepressure chambers 6 and 8 are connected via control connections 11 and13, respectively, to a pilot valve 9 which, in turn, can be actuated bymeans of a control electronics unit 15 which is formed in a conventionalmanner and is therefore not illustrated and described in detail. Thecontrol electronics unit 15 is accommodated in the housing of aninductive travel sensor 17 which is attached to the valve housing 3 andvia the control electronics unit 15 forms part of a positioning controlcircuit determining the axial position of the control piston 5.

That section of the control piston 5 which extends forward from thecontrol collar 4, i.e. out of the valve housing 3, forms a pipe valve 10which is guided in a valve bushing 12. The open, front end of said pipevalve forms the consumer connection A, while the pressure-oil connectionP and tank connection T are offset inward. At an appropriate, axialposition of the control piston 5, the first control aperture 16 andsecond control aperture 18 produce the connection to the pressure-oilconnection P and to the tank connection T, respectively.

At its rear, inner end, the control piston 5 has equalizing channels 21which form a connection to the inner pressure-equalizing chamber 23 inthe valve housing 3. At the rear end, the control piston 5 is connectedto a coaxial directional transmitter rod 25 which extends into a travelsensor tube 27 of the inductive travel sensor 17. A compression spring29 is clamped between the rear end of the control piston 5 and theopposite wall of the pressure-equalizing chamber 23, said compressionspring prestressing the control piston 5 for movement forward, i.e. outof the valve housing 3 (to the right in the drawing). If there is noactivation, i.e. in an operating state in which there is no controlpressure which is crucial for the position of the control piston 5, i.e.there is no difference in pressure between the pressure chambers 6 and8, the actuating force of the compression spring 29 causes axialdisplacement of the control piston 5 into a position in which thecontrol edges of the control apertures 18 form a connection from theinterior of the pipe valve 10 to the tank connection T.

A resultant pressure-medium flow, which flows via the consumerconnection A into the pipe valve 10 from the front end, exerts,depending on the strength of the pressure-medium flow, correspondingflow forces on the control piston 5, said forces counteracting thespring force of the compression spring 29 whose spring force is selectedin such a manner that even pressure-medium flows of average strength aresufficient to prevent the compression spring 29 from greatly opening theconnection from connection A to connection T. A cessation of the controlpressure therefore leads only to a gradual reduction in pressure by acorresponding pressure-medium flow from connection A to connection T.

Instead of a spring arrangement as is realized in the exemplaryembodiment by the compression spring 29, another type of actuator actingon the control piston 5 could be provided, for example an arrangementacting mechanically or electromagnetically via the directionaltransmitter rod 25. While the exemplary embodiment which is illustratedconcerns a three-way directional control valve, it goes without-sayingthat it may also be advantageous in other types of valve, for example2-way directional control valves, to ensure, when there is noactivation, a defined operating state by means of an actuating forceacting independently of the control pressure.

What is claimed is:
 1. A directional control valve comprising; of theactuating force is selected to be just sufficient so that the flowforces compensate for action of the actuating force on the controlpiston (5).
 2. The directional control valve as claimed in claim 1,wherein the control piston (5) forms, at a front end of said controlpiston (5), a pipe valve (10) which is guided in a valve bushing (12)provided at a relevant end of a valve housing (3), wherein an open,front end of the pipe valve (10) is connected to the consumer connection(A), and wherein the pipe valve (10) has at least one control aperture(18) which produces the connection to the tank connection (T) in anaxial position which is slid forward out of the valve housing (3). 3.The directional control valve as claimed in claim 2, wherein theprestressing force means comprises a spring (29) for producing theactuating force.
 4. The directional control valve as claimed in claim 3,wherein the spring is a compression spring (29), said compression springbeing supported at one end on a rear end of the control piston (5) andanother end on an inner end of an interior of the valve housing (3). 5.The directional control valve as claimed in claim 1, wherein theprestressing force means comprises a spring for producing the actuatingforce.
 6. The directional control valve as claimed in claim 5, whereinthe spring is a compression spring (29), said compression spring beingsupported at one end on a rear end of the control piston (5) and atanother end on an inner end of an interior of the valve housing (3). 7.A directional control valve comprising: a valve housing with twopressure control chambers, a movable control piston disposed within thevalve housing and having a control collar which separates the twopressure chambers from each other, which pressure chambers aresubjectable to a control pressure to move the control piston along anaxis of the valve housing into axial positions corresponding torespective ones of a plurality of switch positions of the valve inaccordance with a difference in pressure prevailing between the twopressure chambers; a system for applying an actuating force based on adifference in pressure between the two chambers upon the control piston,and for applying a prestressing force upon the control piston formovement into one of said axial positions wherein, in an absence of adifference in pressure between the two pressure chambers, saidprestressing force positions the control piston for coupling fluid viaan aperture of the valve housing in one of said switch positions, thecoupling of fluid in said one switch position introducing an axiallydirected fluid force counteracting said prestressing force; wherein saidforce applying system exerts an actuating force on the control pistonthat counteracts forces of the fluid which act on the piston during aflow of pressure medium from a consumer connection of the valve housingto a tank connection of the valve housing, and wherein an amount of theactuating force is selected to be just sufficient so that the flowforces compensate for action of the actuating force on the controlpiston.(5).
 8. A valve according to claim 7, wherein said system forapplying the actuating force includes a spring located between saidpiston and said housing for providing said prestressing force.